1. Field of the Invention
The present invention relates to a hollow oxide particle and a process for producing the same. The hollow oxide particle is, for example, applicable to catalytic carriers.
2. Description of the Related Art
There has been provided a hollow oxide particle whose specific surface area is generally 5-10 times or more times larger than that of a solid powder particle, a non-hollow particle. The hollow oxide particle is larger in diameter than the non-hollow powder particle, when the specific surface areas of both particles are same. Such particle having larger diameter is treated easily. So, the hollow oxide particle is applicable to catalytic carriers because of their large specific surface area and large diameter. It is also expectable as concealing agents and micro-capsules.
Japanese Unexamined Patent Publication (KOKAI) 6-7,670 discloses a process for producing a porous particle or a hollow particles formed. In this publication technique, firstly, spherical polymer particles for forming cores are uniformly dispersed in a solution including a metal salt having a hydrolysis property, so that polymer-metal compound particles are formed in which each spherical polymer core is covered with a metal compound layer. The polymer-metal compound particle is in the range of 0.07-30 .mu.m in diameter, and its ratio of inner-diameter to outer-diameter is in the range 0.40-0.95. In this publication technique, secondly, the polymer-metal compound particles are heated for decomposing the core-polymer into carbon or carbon dioxide, so that they are transformed into another spherical particles in which each spherical carbon is covered with a metal compound layer or a hollow particle. This publication technique exhibits complicated steps to increase costs. Also, this publication technique, using a precipitating method, can hardly produce a homogeneous complex oxide particle because the precipitation rates of the metal salts are usually different from each other.
Japanese Unexamined Patent Publication (KOKAI) 63-258,642 discloses a process for producing a hollow particle. In this publication process, after an Oil in Water (O/W) type emulsion is composed by mixing an organic solvent with an aqueous solution including inorganic compounds, the Oil in Water (O/W) type emulsion is added into an organic solvent including a hydropholic surfactant to compose an Oil in Water in Oil (O/W/O) type emulsion. Next, the O/W/O type emulsion is mixed with a solution including compounds capable of precipitating a water-insoluble precursor in order to form a hollow particle. This publication process requires a calcination step of the precipitated precursor to cause complicated steps, the process exhibits a problem in homogeneity of the particle, and it can not produce a hollow particle having a shell wall thickness of 20 nm or less.
Japanese Unexamined Patent Publication (KOKAI) 3-47,528 discloses a process for producing spherical a hollow particle of metal oxide. In this publication process, firstly, an emulsion is composed by a mix of a first oil and a second oil into a metal compound solution. Secondly, the spherical hollow particle is made by removing of the oils and water from the emulsion. This publication process causes complicated steps, it can hardly produce a homogeneous complex oxide particle because the solubilities of metal compounds are different from each other, and it can not produce the particle whose shell wall has a thickness of 20 nm or less.
Japanese Unexamined Patent Publication (KOKAI) 60-122,779 discloses a technique for producing porous particles. This technique uses MgAl.sub.2 O.sub.4 obtained by a spray pyrolisis as a raw material. The raw material is formed and sintered to produce the porous body in which the pore size and distribution are easily controlled. In this technique, each of atomized droplets, the diameter of which is in the range of tens .mu.m in size under efficient conditions, constitutes a reaction site. Namely, size of the reaction site of this technique is ten times or much larger than that of an emulsion-combustion method. This technique, therefore, may cause heterogeneous particles, and it has difficulty in producing the hollow particle having a shell wall thickness of 20 nm or less, judging from the aforementioned presupposition that each of the atomized droplets constitutes each of porous particles.
Japanese Unexamined Patent Publication (KOKAI) 8-91,821 discloses a technique for producing a hollow silica particle by use of a sol-gel method of alkoxide. This technique uses alkoxide raw material exhibiting expensive costs. With this publication disclosing nothing about a shell wall thickness, judging from the fact that the hollow silica particle is as large as 2-8 .mu.m in diameter, it is hardly thought that the hollow silica particles has a shell wall thickness of 20 nm or less.
Further, "Ceram. Inter." (vol.,14 (1988), 239-244) discloses a technique for producing a hollow alumina particle by use of an emulsion-evaporating technique. This reference technique requires a calcination step after producing a precursor. According to a Scanning Electron Microscopy (SEM) photograph in this reference, a shell wall of the particle is hundreds nm or more in thickness.